Microcalorimetry was used to measure the change in enthalpy for the scission of calf thymus DNA (ct-DNA) induced by adriamycin (ADM) in the presence of ferric ions, Vitamin C, and oxygen. At 298.15K and pH 7.4, the overall molar reaction enthalpy for this cleavage was-147.1kJ/mol, noticeably higher than that by the mixture of Fe3á, Vitamin C, and O2. Under the same conditions, the enthalpy change for the damage of ct-DNA by the mixture of adriamycin, ferrous ions, and oxygen, however, was nearly zero, indicating that this mixture can not induce any detectable degradation of DNA. These esults suggest that both the activated adriamycin and hydroxyl radical attack DNA strands during the cleavage. A possible mechanism for the cleavage of DNA induced by adriamycin is proposed based on the calorimetric measurements. A novel thermodynamic model for the interactions of DNA with small molecules is also suggested. This is a convenient method tocalculate both the binding constant (Kb) and the standard thermodynamic parameters (ΔbH0m, ΔbG0m, and ΔbS0m) for the binding of adriamycin-Fe3á complex to ct-DNA by the calorimetric data. This nucleotide binding reaction is driven by a favorable enthalpy change, with a large unfavorable entropy change. This result indicates that the binding results in structural changes accompanied by an increase in the order of the whole system, implying that an intercalation mode is involved in adriamycinmediated breakage of DNA.